We use the ARP Head Stud Kit on every engine we build and race
because in heavy use applications, stock parts don't get the job done. Simply
stated, there's not a better stud setup on the market today.
For starters, ARP Head Stud Kits use a premium grade 8740
alloy that is rated far superior to even aircraft-quality parts. Then, each stud is
placed vertically in special racks and precisely heat-treated to 200,000 psi.
This procedure ensures complete heat penetration, and the results are far
superior to those lesser quality studs from other manufacturers who just dump
pieces in a basket and hope for the best.
Following the heat treatment, each ARP Head Stud is centerless
ground to make it as close to perfectly concentric as possible. This procedure
involves about ten very slight cuts and results in an exceptionally straight
part. It's important to note that lesser quality studs are not even centerless
ground - the material is thread rolled in bar stock form, and usually before
heat treating, when the material is easier to machine.
ARP Head Stud Kits are thread rolled after heat treating,
which gives them about 1000% (that's ten times) better fatigue strength than
those studs that are threaded prior to heat treatment (a very common industry
practice). It costs more to do it this way, because it's tough on tooling,
but the results are well worth the extra effort.
You will also note that ARP offers specially undercut
studs for several engines. This procedure (done only to the shorter studs) more
equalizes the "stretch" of both studs, which makes for a more consistent
clamping force - and one that compensates for head gasket compression when the
cylinder heads are installed. This helps prevent blown head gaskets and assures
optimum engine sealing!
Because ARP Head Stud Kits are manufactured to such exacting tolerances, you will note that gaskets and cylinder heads literally glide into position and are perfectly aligned - something that won't happen with inferior-quality head studs and bolts.
Head Studs vs. Bolts - ARP Technical Discussion
ARP's factory Tech Representatives are often asked which
is better, cylinder head studs or bolts. The answer, invariably, depends on the
installation. On many street-driven vehicles, where master cylinders and other
items protrude into the engine compartment, it's probably necessary to use head
bolts so that the cylinder heads can be removed with the engine in the car.
For most applications, however, studs are recommended. And
for good reason. Using studs will make it much easier to assemble an engine
(especially a racing powerplant which must be serviced frequently and quickly!)
with the cylinder head and gasket assured of proper alignment. Studs also
provide more accurate and consistent torque loading.
Here's why. When you use bolts to secure the head, the
fastener is actually being "twisted" while it's being torqued to the proper
reading. Accordingly, the bolt is reacting to two different forces
simultaneously. A stud should be installed in a "relaxed" mode - never crank it
in tightly using a jammed nut. If everything is right, the stud should be
installed finger tight. Then, when applying torque to the nut, the stud will
stretch only on the vertical axis. Remember, an undercut shorter stud will have
a rate similar to a longer, standard shank stud. This provides a more even
clamping force on the head. Because the head gasket will compress upon initial
torquing, make sure studs and bolts are re-torqued after the engine has been
run.
Additionally, in BMW applications where factory head bolts
are usually stretch bolts, ARP head studs do not require replacement after each
use. This is a decided cost advantage for performance engines which
undergo occasional inspection.
BMW Fitments:
2 Series
Vintage 2002 (67-76) > E10 2002 (1969-1976)
Vintage 2002 (67-76) > E10 2002tii (1972-1974)
3 Series
E21 (75-83) > 320i
3 Series
E30 (84-91) > 318i